/Engine/Plugins/Media/MediaIOFramework/Source/MediaIOCore/Public/MediaCapture.h
Callback when the source texture is ready to be copied on the GPU. You need to copy yourself. You may use the callback to add passes to the graph builder. Graph execution and output buffer readback will be handled after this call. No need to do it in the override. The callback in called from a critical point be fast. The texture is valid for the duration of the callback. Only called when the conversion operation is custom and the output resource type is Buffer.
virtual void OnCustomCapture_RenderingThread ( [FRDGBuilder](API\Runtime\RenderCore\FRDGBuilder) & GraphBuilder, const [FCaptureBaseData](API\Plugins\MediaIOCore\UMediaCapture\FCaptureBaseData) & InBaseData, [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [FMediaCaptureUserData](API\Plugins\MediaIOCore\FMediaCaptureUserData), ESPMode::ThreadSafe > InUserData, FRDGTextureRef InSourceTexture, FRDGBufferRef OutputBuffer, const [FRHICopyTextureInfo](API\Runtime\RHI\FRHICopyTextureInfo) & CopyInfo, FVector2D CropU, FVector2D CropV )